In modern electronics, phase shifters are used in many applications. Especially in communication electronics phase shifters are used to modify transmitted or received signals. In wireless communication, phase shifters are used in phasing the input signals of antenna arrays so that different transmission beams may be formed.
In general, a phase shifter receives as an input an electric signal and produces at the output two signals which are shifted with respect to each other by a predetermined phase angle.
In many applications, a constant phase shift over a wide frequency band is desired. This applies especially to communication applications. Recently, solutions where combined backward and forward transmission lines are used have been suggested (M. A. Antoniades and G. V. Eleftheriades. Compact Linear Lead/Lag Metamaterial Phase Shifters for Broadband Applications. IEEE Antenn. Wireless Propag. Lett., 2:103, 2003).
Solutions where a phase shift is produced by a simple forward transmission line show a linear dependence on frequency. A simple backward transmission line is generally even more dispersive in frequency domain. In proposed solutions where combined forward-backward transmission lines are used, frequency dependence has reduced remarkably. However, the solutions are restricted to phase shifts around 180°, and cannot be used for creating arbitrary phase shifts.